Being the speed of light a finite value, watching the stars is like taking a trip to the past. This is not only a fascinating and romantic fact but it’s also a great opportunity for astronomers that, studying deep sky objects, can understand how our Solar System evolved. Doing so, it’s not rare that “strange” things are discovered, and this is the case of what has been found by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile: Trojan bodies in an exoplanetary system.
What are Trojan objects? Why is this discovery so important?
Trojan objects are not so unusual indeed. They are objects that share the orbit with a major body and are well-known inside the Solar System. The first Trojan to be discovered (588 Achilles) was an asteroid that share its orbit with Jupiter and was spotted for the first time in 1906. Since then, thousands of Trojans have been found orbiting together the planets of our System, even the Earth has two (2010 TK7 and 2020 XL5).
This time, however, ALMA searched beyond the boundary of the Sun’s gravitational influence. In particular, astronomers focused their efforts on the star PDS 70, part of the constellation Centaurus. This is a unique target since it’s home to two protoplanets (very young planets) surrounded by a protoplanetary disk (disk of dense gases surrounding a young star). Through a reanalysis of public data from ALMA, astronomers found a cloud of debris orbiting around one of the two newborns (PDS 70b). The peculiarity of this cloud is that has a mass around two times the one of the Moon.
In a study published on Astronomy&Astrophysics, the team studying this system proposed some possible explanations. In particular, they believe this could be an existing Trojan world or a new planet forming. If this last hypothesis turns out to be correct, it would be a mind-blowing discovery: it would be the demonstration that two planets could share the same orbit!
In any case, however, the presence of Trojan bodies in other stellar systems would support the hypothesis of them being a common consequence of planetary formation.
The Atacama Large Millimeter/submillimeter Array (ALMA)
Crucial to this discovery was the telescope ALMA. This is not a telescope like the ones that populate the collective imagination, with lenses or mirrors. It is a group of 66 radio telescopes able to capture light wavelengths of around one millimeter, allowing them to see clouds in interstellar space with a temperature just above absolute zero but also some of the first galaxies born. These appear to be dark if observed in visible light but are very bright between infrared and radio wavelengths.
This powerful instrument was the best option to search for exotrojan objects thanks to its high resolution (ten times more accurate than the one of the Hubble Space Telescope) and the ability to see very cold objects.
To confirm what ALMA observed, however, astronomers need to wait until 2026 when the PDS 70b orbit will be favorable again to study the co-orbital motion. If confirmed, this will probably be only the first discovery of its kind since ALMA is planned to be upgraded, allowing it to perform this kind of “hunting” in a more efficient way!